Process plants, such as refineries, chemical plants or pulp and paper plants, consist of numerous process control loops connected together to produce various products. Each of these process control loops is designed to keep some process variable such as pressure, flow, level, or temperature, within a required operating range to ensure the quality of the end product. Control valves or control regulators within the system manipulate a flowing fluid, such as gas, steam, water, or a chemical compound, to compensate for load disturbances and to maintain regulated process variables as close as possible to a desired control or a set point.
Various control valve configurations may be applicable for certain applications. For example, when a quick-opening valve with a narrow control range is suitable, a rotary control valve, such as a butterfly valve, may be used. Alternatively, when precise control over a large control range is required, a sliding stem control valve may be used. In any configuration, such control valves are generally coupled to a control device such as an actuator, which controls the exact opening amount of the control valve in response to a control signal.
In some systems, especially in pneumatically controlled fluid process systems, the actuator for any given fluid process control device may include a diaphragm actuator. Typical diaphragm actuators comprise a housing containing a spring-biased diaphragm assembly. The diaphragm assembly is operatively coupled via a stem, or other actuator rod, to a valve plug, or other control member, to control the opening amount of the fluid process control device.
Some assemblies include a diaphragm and one or more diaphragm plates. The diaphragm includes a flexible disk-shaped member. The plate or plates are disposed adjacent to the diaphragm and are adapted to be engaged by one or more springs disposed within the housing. Additionally, the plates provide a rigid mechanical connection to the stem. The springs serve to bias the diaphragm assembly into a predetermined position such that the actuator may bias the control device into an open or closed configuration. In some devices, the diaphragm plate includes a dished portion, against which the diaphragm is sealed with a standard worm gear hose-clamp. In other devices, the diaphragm is not fixed to the plate at all. However, the one or more diaphragm plates, as mentioned, are rigidly fixed to the stem of the actuator. Such fixation is generally achieved by threaded attachment. For example, in one form, the stem includes a threaded end portion disposed through a central aperture in the one or more plates. A nut is then threaded onto the threaded end portion of the stem to attach the stem to the plate(s).
When the actuator is exposed to excessive fluid pressure, such as when a supply regulator fails, actuator components are subject to failure, leading to a loss of proper process control and/or to a dangerous situation where high pressure fluid may be sprayed into the environment. To prevent catastrophic failures, some high pressure systems include an external pressure relief valve or a rupture disc to relieve pressure when the pressure exceeds a maximum set point. However, these external pressure relief valves and/or rupture discs add additional costs to the system and are often not used due to the additional costs.